linux-kernel - Re: [PATCH v3 1/3] dt-bindings: phy: qcom,qmp: Add PCIe qcom,refclk-always-on property

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Date:   Thu, 7 Dec 2023 18:50:32 +0530
From:   Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To:     Johan Hovold <johan@...nel.org>
Cc:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Krishna Chaitanya Chundru <quic_krichai@...cinc.com>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Vinod Koul <vkoul@...nel.org>,
        Kishon Vijay Abraham I <kishon@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        linux-arm-msm@...r.kernel.org, linux-phy@...ts.infradead.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        quic_vbadigan@...cinc.com, quic_ramkri@...cinc.com,
        quic_nitegupt@...cinc.com, quic_skananth@...cinc.com,
        quic_vpernami@...cinc.com, quic_parass@...cinc.com
Subject: Re: [PATCH v3 1/3] dt-bindings: phy: qcom,qmp: Add PCIe
 qcom,refclk-always-on property

On Thu, Dec 07, 2023 at 02:05:12PM +0100, Johan Hovold wrote:
> On Thu, Dec 07, 2023 at 03:42:52PM +0530, Manivannan Sadhasivam wrote:
> > On Thu, Dec 07, 2023 at 10:51:09AM +0100, Johan Hovold wrote:
> > > On Wed, Dec 06, 2023 at 06:40:09PM +0530, Manivannan Sadhasivam wrote:
> > > 
> > > > OK. How about, "qcom,broken-refclk"? This reflects the fact that the default
> > > > refclk operation is broken on this platform, so the OS should be prepared for
> > > > it (by keeping it always on).
> > > 
> > > Shouldn't that be
> > > 
> > > 	qcom,broken-clkreq
> > > 
> > > since its the CLKREQ# signal used to request REFCLK that is broken, not
> > > the REFCLK itself?
> > > 
> > 
> > Darn... You are right. I got carried away by the initial property name. Thanks
> > for spotting!
> 
> Thinking some more on this after hitting send: It may still be wrong
> with a 'broken-clkreq' property in the PHY instead of in the controller
> (or endpoint).
> 
> Could there not be other ways to handle a broken clkreq signal so that
> this really should be a decision made by the OS, for example, to disable
> L1 substates and clock PM?
> 

One has to weigh the power consumption between keeping refclk always on and
disabling L1SS. Chaitanya, can you measure power consumption in both cases?

> Simply leaving the refclk always on in the PHY seems like a bit of a
> hack and I'm not even sure that can be considered correct.
> 

I wouldn't agree it is a hack, even though it may sound like one. The option to
keep refclk always on in the PHY is precisely there for usecase like this.

- Mani

> Having a property that maps directly to that behaviour has rightly been
> rejected, but it seems that simply renaming the flag but keeping it in
> the PHY may still not be the right thing to do here.
> 
> Johan

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